Gold and silver are usually extracted from their ores by a leaching process. The ores may be specifically mined for their gold or silver content, or the metals may be obtained as by-products when the base metals such as lead, zinc and copper are refined.
The auriferous ore undergoes a primary and secondary crushing to size of about 3 mm. This is followed by a fine grinding, for example in a ball mill to a particle size of about -0.074 mm (-200 mesh). The lixiviant is mixed with the ore during and/or after fine grinding. Suitable lixiviants for the extraction of gold, include dilute alkaline solutions of sodium cyanide and potassium cyanide. Alkaline cyanide solutions are the preferred lixiviants in the current commercial practice.
The cyanide solution is used in dilute form, for example 0.025% by weight, with protective alkalinity e.g. by the addition of lime, and is added in an amount to give a slurry generally having about 40 to 50% by weight solids content. This slurry is held in air-agitated pachuca tanks for periods up to 50 hours to achieve maximum extraction of gold. The gold present in the ore reacts with the cyanide solution to yield the water soluble complex. If sodium cyanide is used as the lixiviant Na(Au(CN).sub.2) is formed. The suspension is agitated with compressed air during the extraction.
The solution of the gold compound is then filtered and de-aerated, for example, under vacuum to remove the dissolved air. The metal is then recovered from solution by zinc cementation or by electrolysis. In the case of zinc cementation a suspension of finely powdered zinc dust is added to the solution and the gold precipitated and separated by further purification.
In the case of the extraction of silver with cyanide solution a reaction occurs between free silver and, for example, the silver sulphide present in argentite, and the cyanide solution to yield the water soluble argentocyanide e.g. Na(Ag(CN).sub.2) and the slurry is agitated with compressed air. The recovery of the metal from the silver containing solutions is achieved in a similar manner to that of gold.
The above process suffers from the following disadvantages. The ore must be ground to a fine particle size which is both expensive and time consuming. The slurry leach process may take several hours to extract the metal and the filtration of the resulting gold bearing solution may be arduous due to the fine particle size of the filtered solids.
The present invention has been made with the above points in mind.